This invention relates generally to sonar baffles and more particularly to compliant baffles.
As is known in the art, a baffle is a device that acts as a partition for preventing interference between sound waves in separate, adjacent enclosures. In general, sonar baffles are energy canceling or energy absorbing devices in which undesired acoustic noise is reduced by such mechanisms as shear and/or torsion absorption or cancellation of the signals by the generation of equal signals that are out of phase with respect to signals received by the baffle. In sonar systems, baffles are typically used to isolate highly sensitive acoustic receivers from undesired acoustic signals, generally referred to as noise. In such systems, it is generally desired to increase the overall response of a sonar system to a desired signal by decreasing the response of the system to the undesired noise.
In many applications, passive sonar receiving systems, such as hydrophone arrays, are located on moving vessels for detecting acoustic signals propagating in the ocean. Noise generated on or by the moving vessel, such as machinery noise, hydrodynamic noise, and the noise produced by the activities of the vessel's crew is typically called "self noise" and can mask a desired signal received by the hydrophone array. Moving vessels such as submarines and ships have hulls generally constructed of large metal plates mounted to bulkheads. The metal plates are easily excited into flexural vibration by the on-board machinery and hydrodynamic noise and act as an acoustic radiator to the ocean medium. In one application, a sonar baffle assembly is placed between a highly sensitive hydrophone array and the hull of a ship or submarine. The baffle minimizes the acoustic energy propagating from the hull structure by absorbing the acoustic energy within the sonar baffle and/or by reflecting the acoustic energy back to the hull. Some of the sources of radiated noise produce a line-component spectrum in which the noise is dominated by tonal components at a fundamental frequency and related harmonics of the vibration producing process. Other sources of radiated noise produce a generally continuous spectrum related to the excitation of structural members, such as the hull, into resonance. This type of noise, sometimes called high wavenumber noise, does not generate propagating acoustic waves and is referred to as evanescent in nature. Accordingly, while this slow-wave, non-propagating noise is unlikely to be detected by an unfriendly listener some distance away, it is likely to overload the output of an adjacent hydrophone array used to detect acoustic signals propagating in the ocean.
There are a wide variety of sonar baffles having different sizes, shapes, and material compositions generally dependent on the particular application and the frequency of operation.
One sonar baffle configuration used in underwater environments is the compliant plate baffle. The compliant plate baffle typically has a pair of flexible metallic or composite material plates coupled together with ball joint hinges at each end of the plates. The plates have lengths chosen to resonate at frequencies typical of the undesired acoustic signals and are generally separated by a nylon insert for dampening the resonant acoustic signals. The hinged flexible plate assemblies are generally embedded in a rigid polymer plastic material such as polyurethane for protection against the corrosive effects of salt water. Acoustic signals incident on the compliant baffle assembly pass easily through the polyurethane and resonate the hinged plates. Although compliant plate baffles provide some absorption to incident acoustic signals, ordinarily, compliant baffles are designed to cancel the incoming acoustic signals. Acoustic signals incident on the baffle assembly cause the plates to contract and then expand back to their original shape, releasing a pressure wave at the frequency of the incident signal but with a differential phase shift. This response will generally provide partial cancellation of the incident acoustic signals thereby providing isolation to the receiver. Due to the inherent size and strength of the materials used in their construction, the compliant plate baffle provides high insertion loss to undesired acoustic signals and can withstand high hydrostatic pressure characteristics typical of deep ocean depths. However, compliant plate baffle manufacturing costs are relatively high, compared with other baffle designs, due to the large number of component parts required in each assembly.
Another baffle configuration used in underwater systems is the compliant oval-shaped tube baffle. Compliant oval-shaped tube baffle assemblies include a plurality of elliptically shaped or oval flexible tubes generally fabricated of metal or composite materials. Compliant oval-shaped tube baffles are pressure or energy canceling devices and operate in the same way as compliant plate baffles. The compliant oval-shaped tubes are generally encapsulated and are generally uniformly oriented such that the minor diameters of the elliptical tubes essentially define the thickness of a layer of the baffle. Encapsulation of the baffle assembly is generally undesirable since this process may allow the generation of other modes and provide absorption of the incoming signal which reduces the peak insertion loss of the device. However, encapsulation is usually required for protecting the baffle against the corrosive effects of saltwater and industrial solvents and for ease of handling the assembly.
One problem with the compliant oval-shaped tube baffle having uniformly oriented oval-shaped tubes is that within the band of operation, certain frequencies excite the oval-shaped tubes in what are known as antisymmetric modes. Acoustic signals at these frequencies pass through the baffle with little attenuation. The theory of acoustic frequency "shorts" due to resonance and coupling of antisymmetric tube modes has been published in an article entitled "Scattering by multiple gratings of compliant tubes" by Radlinski and Janus, J. Acoust. Soc. Am. 80(6), December 1986.
Another problem with the compliant oval-shaped tube baffle relates to its displacement profile during excitation. In operation, the excited oval shaped compliant tube has portions which compress the surrounding medium while other portions of the tube concurrently rarefy the medium. In such situations, the displacement pattern is said to have both positive and negative displacements. In the case of the oval compliant tube, during the first half cycle of excitation, the vertices of the oval tube have a negative displacement, while the broad walls of the tube are providing positive displacement. This effect generally reduces the complaint oval tube's efficiency in reflecting back the incident wave. This effect also provides additional mechanical stress in the tube. The measure of positive displacement in relation to concurrent negative displacement is called volume flow. Baffles having low volume flow store less acoustic energy, reradiate less energy and accordingly, have reduced insertion loss characteristics.
Further, because the compliant oval-shaped tube baffles are fabricated from continuous hollow oval tubes and are uncompensated, circumferential stresses due to hydrostatic pressure can be significant. These stresses are related to the bending stiffness characteristic of the tube and its geometry.
Both compliant tube and compliant plate baffles have the additional problem of having reduced effectiveness in attenuating small size flexural wavelength noise. This ineffectiveness is related to the large size of the compliant devices relative to the evanescent wavelength. The short wavelengths of the flexural noise appear almost transparent to the much larger compliant tubes and plates.
Still another baffle commonly used when light-weight, smaller size, easy installation and maintenance, and low cost are needed, is the voided elastomer or so-called "air/rubber" baffle. The air/rubber baffle generally has a construction that includes sheets of energy absorbent material, such as rubber, having an arrangement of densely packed air-hole pockets disposed within each sheet. The diameters of the air-hole pockets are predetermined to limit the transmission of the undesired acoustic waves incident upon the baffle. Although the air/rubber baffle does not provide insertion loss of the magnitude typical of the costlier and heavier compliant plate baffle, the simplicity of its design makes it popular for use for many applications.
One problem with the air/rubber baffle is its relative ineffectiveness at ocean depths where high hydrostatic pressure conditions exist. The problem of hydrostatic pressure loading is not unique to sonar baffles, and commonly changes the operating characteristics of many components (e.g. transducers) used in sonar systems. In the case of an air/rubber baffle, these high pressure conditions can compress the energy absorbent sheets to the extent that they no longer absorb acoustic energy incident upon them, or at the very least, physically change the geometry of the air-hole pockets such that undesired acoustic signals propagate through the baffle without being attenuated.
In addition, the air/rubber baffle is generally unsuitable for use in environments where underwater explosions can occur. In these situations, very high hydrodynamic pressure conditions provide pressure levels beyond the strength capabilities of the elastomer material. Cracks generally form in the material which allow the air-hole cavities to fill with water, resulting in an inoperable sonar baffle.
In the application described previously, a sonar baffle assembly is placed between a hydrophone array and a hull structure. In this type of application, compliant plate baffle assemblies are typically disposed directly beneath the hydrophone array to take advantage of their high insertion loss characteristics, while voided elastomer baffles or compliant oval-shaped tube baffles are disposed adjacent to and around the perimeter of the compliant plate baffle assemblies.